The present invention relates to a mold assembly for molding a thermoplastic resin and a method of manufacturing a molded article of a thermoplastic resin by means of the above mold assembly. More specifically, it relates to a mold assembly which serves to manufacture a molded article having an improved transferred surface and having no defect in appearance by an injection molding method, an injection compression molding method, a blow molding method or the like, and a method of manufacturing a molded article of a thermoplastic resin by means of the above mold assembly. Or, it relates to a method of manufacturing a molded article of a thermoplastic resin, which method can effectively prevent occurrence of a sink mark on the molded article.
Generally, a mold used for manufacturing a molded article of a thermoplastic resin (to be simply referred to as "mold" hereinafter) is formed from a metal material that is not deformed under a high pressure exerted at the time of injecting or introducing a molten thermoplastic resin (to be sometimes simply referred to as "molten resin" hereinafter) into a cavity , i.e., a hollow portion, provided in the mold, or filling the cavity with the molten resin. The metal material is selected, for example, from carbon steel, stainless steel, aluminum alloy or copper alloy. Injecting or introducing a molten resin into a cavity provided in a mold or filling a cavity with a molten resin is sometimes simply referred to as "introducing a molten resin into a cavity" hereinafter. A molten resin is introduced into the cavity, and as a result, a molded article having a desired form and having a surface on which a surface of the mold is transferred is obtained. The surface of the mold which constitutes the cavity is sometimes referred to as "cavity wall of the mold" hereinafter.
A thermoplastic resin is generally classified into two types of resins such as an amorphous thermoplastic resin and a crystalline thermoplastic resin. An amorphous resin starts to be solidified or softened at its glass transition temperature T.sub.g. A crystalline thermoplastic resin is melted at a temperature equal to, or higher than, its melting point T.sub.m. and a crystal is formed and grown at a temperature equal to, or lower than, its crystallization initiation temperature T.sub.c, and the crystalline thermoplastic rein is solidified.
When a molded article is manufactured by means of the above mold made of a metal, it is difficult to form the molded article having a surface state similar to that of a cavity wall of the mold. Generally, a mold is made of a metal material which is not deformed by a high stress brought by a pressure of an introduced molten resin, and the metal material has an excellent thermal conductivity. Therefore, as soon as the molten resin introduced into the cavity contacts the cavity wall of the mold, it starts to be instantaneously cooled. As a result, a solidification layer is formed in a molten resin portion which contacts the cavity wall of the mold, the molded article is liable to have defects in appearance such as a weld-mark or a flow mark, and there is also a problem that the poor transfer of the cavity wall of the mold on the surface of the molded article takes place. In a molded article having a projection such as a lib or boss (thick wall portion), further, a sink mark is often formed on a portion of one surface of the molded article which portion is opposed to the projection (a lib or boss) formed on the other surface of the molded article.
For overcoming the above-explained various problems, generally, there is employed a method in which a cavity wall of a mold is forced to be transferred on a surface of a molded article by injecting a molten resin into a cavity under high pressure, or a method in which development of a solidification layer of a thermoplastic resin is delayed by setting the mold temperature at a high level, for preventing occurrence of a weld-mark or a flow mark and for preventing occurrence of poor transfer of the cavity wall of the mold on the surface of the molded article. However, the former method has the following problem; the molding machine is increased in size, and the mold itself is increased in size and thickness, all of which require additional costs. Further, the molded article has a residual stress inside due to the introduction of the molten resin into the cavity under high pressure, and as a result, the molded article is poor in quality. On the other hand, the latter method has the following problems. Since the mold temperature is set at a level close to and lower than the deflection temperature of the used thermoplastic resin under load for preventing the development of the solidification layer, a longer period of time is required for cooling the resin in the cavity. As a result, the molding cycle takes a long time, and the productivity decreases. Further, there is another problems that since the mold temperature is high, the shrinkage amount of the thermoplastic resin increases at a cooling time and a sink mark is much more increased in size and/or depth.
For preventing appearance defects of a molded article such as a weld-mark and a flow mark caused by the development of the solidification layer in that portion of the molten resin which contacts to the cavity all of the mold and the poor transfer of the cavity all of the mold on the molded article, for example, JP-A-55-55839, JP-A-61-100425, JP-A-62-208919, JP-A-5-111937, JP-A-5-200789, JP-B-6-35134 and JP-A-6-218769 disclose methods in which a member having a low thermal conductivity is provided or attached to a cavity wall of a mold in order to delay the development of a solidification layer and prevent molding defects such as a weld-mark and a flow mark.
However, the method of simply attaching the member having a low thermal conductivity to a portion of the mold with an adhesive has the following problems, and the mold as a whole is poor in durability, so that it is difficult to mass-produce molded articles.
(1) When a clearance between the member having a low thermal conductivity and an attaching portion of the mold where the member having a low thermal conductivity is attached is small, the insert block is broken by repeating an increase and a decrease in the temperature of the mold since the coefficient of linear expansion differs between the material for constituting the mold and the material for constituting the member. Further, the member having a low thermal conductivity may be broken since it suffers a stress caused by an injection pressure.
(2) When a clearance between the member having a low thermal conductivity and an attaching portion of the mold where the member having a low thermal conductivity is attached is large, a molten resin penetrates a space between the attaching portion and the member in the molding operation, and mold flashes occur on a molded article. Further, when the molded article is released from the mold, the peripheral portion of the member having a low thermal conductivity suffers resistance, and as a result, the member having a low thermal conductivity may be broken.
The method of delaying the development of a solidification layer by using a member having a low thermal conductivity, which is formed of a heat-resistant plastic, has the following problems. Since such a member has low rigidity and has a poor surface hardness, the member having a low thermal conductivity is deformed and damaged when used for a long period of time. There is another member having a low thermal conductivity, which is prepared by forming a thin film of ceramic or the like on a metal surface by a chemical deposition method. However, such a member has a problem in that the thin film has poor durability and peels off the metal surface. The above members are therefore used in an experimental mold or in a simple mold and are not competent for use for long period of time.
Applicant in Japanese Patent Application No. 7-152519 (JP-A-8-318534, corresponding to U.S. Pat. No. 5,741,446) proposed a mold assembly comprising a mold, an insert block and a cover plate. The above mold assembly is feasible for overcoming the above problems. However, it cannot accommodate the cover plate in a mold depending upon some forms of molded articles, and the position where the insert block can be placed is limited in some cases. That is, in some cases, the insert block cannot be attached to a portion of a mold corresponding to that portion of a molded article which should be imparted with an excellent surface property. For example, when a molded article has an undercut portion, it is required to provide the mold assembly with a slide core for taking out the molded article having an undercut portion from the mold assembly. In the mold assembly having the above structure, however, it is difficult to provide the mold assembly with the cover plate.
A sink mark is formed for one reason that a resin shrinks together with the cooling and solidification of a molten resin, and is therefore a phenomenon which a general injection molding method cannot avoid. For overcoming the above sink mark problem, for example, there has been proposed a method in which a semi-molten thermoplastic resin is compressed at an injection molding time to compensate a shrunk volume of the semi-molten thermoplastic resin, i.e., a method in which the volume of a cavity is decreased by the shrunk volume. There has been also proposed another method of preventing the formation of a sink mark, in which a hollow portion is formed inside a molten resin introduced into a cavity of a mold by introducing an inert high-pressure fluid to exert a pressure on the molten resin from the hollow portion to the cavity wall of the mold. In many cases, however, it is difficult to reliably prevent the formation of a sink mark even if the above methods are employed.